Ch 11: Earth's Interior

Earth’s Interior Layered Structure

The Earth is divided into crust (~8–70 km thick, solid, basaltic–granitic composition), mantle (~2800 km thick, mostly solid ultramafic peridotite composed mainly of olivine and pyroxene), outer core (~2400 km thick, liquid iron-nickel alloy), and inner core (~1220 km thick, solid iron-nickel alloy).

Crust

The outermost solid layer of Earth, 8–70 km thick, composed mainly of basalt (oceanic crust) and granite (continental crust), with P-wave velocities of ~6–8 km/s.

Mohorovičić Discontinuity (Moho)

The seismic boundary between the crust and mantle marked by an abrupt increase in seismic wave velocity due to change from crustal rocks to denser ultramafic mantle rocks.

Mantle

A ~2800 km thick layer beneath the crust composed primarily of ultramafic peridotite (olivine + pyroxene); mostly solid but capable of ductile flow over geologic time.

Outer Core

A ~2400 km thick liquid layer composed mainly of iron and nickel; S-waves cannot pass through it, confirming its liquid state.

Inner Core

A ~1220 km thick solid sphere composed mostly of iron-nickel alloy; solid due to immense pressure despite extremely high temperatures.

Meteorites (Chondrites)

Primitive solar system materials that represent the building blocks of Earth and provide insight into Earth’s bulk composition, particularly iron and nickel abundance.

Iron Meteorites

Meteorites composed mainly of Fe-Ni alloy, used as analogs for Earth’s core composition.

Xenolith

A fragment of mantle rock brought to the surface in volcanic eruptions, providing direct samples of the upper mantle (typically <200 km depth).

Peridotite

Ultramafic mantle rock composed mainly of olivine and pyroxene; represents dominant mantle composition.

Mineral Physics

The study of physical and chemical properties of Earth materials at high pressure and temperature to understand interior processes.

Seismic Wave Velocity

The speed at which seismic waves travel; depends on material composition, density, pressure, and temperature.

Density-Velocity Relationship

In general, denser and stiffer materials transmit seismic waves faster, while hotter and less rigid materials slow them down.

Pressure Effect on Velocity

Increasing confining pressure with depth compresses rocks, making them denser and increasing seismic velocities.

400 km Discontinuity

A seismic velocity jump caused by mineral phase transition (olivine → wadsleyite), marking the top of the mantle transition zone.

660 km Discontinuity

A seismic velocity jump caused by mineral phase change (ringwoodite → bridgmanite + ferropericlase), marking the base of the mantle transition zone.

Transition Zone

Mantle region between ~400 and ~660 km depth characterized by mineral phase changes due to increasing pressure.

Seismic Reflection

The bouncing of seismic waves off a boundary where rock properties change.

Seismic Refraction

The bending of seismic waves as they pass through materials with different velocities.

P-wave Shadow Zone

A region between ~105° and 142° from the epicenter where direct P-waves are not observed due to refraction at the core-mantle boundary.

S-wave Shadow Zone

A region beyond ~105° from the epicenter where no direct S-waves are detected because S-waves cannot travel through the liquid outer core.

Seismic Inversion

A method that uses recorded arrival times of seismic waves to model velocity structures and infer Earth’s interior properties.

Evidence for Iron Core

Earth’s high mass, meteorite composition (Fe-Ni rich), seismic velocity matches to iron alloys, and moment of inertia calculations all indicate a dense iron-rich core.

Geothermal Gradient (Crust)

Typically 20–30 K/km in the crust, controlled largely by conductive heat transfer.

Mantle Geothermal Gradient

Very small (~0.3 K/km) due to efficient convective heat transfer.

Conduction

Heat transfer by molecular vibration and direct contact; dominant in the lithosphere.

Convection

Heat transfer by physical movement of material; dominant in mantle and outer core.

Radiative Heat Transfer

Minor heat transfer mechanism in Earth compared to conduction and convection.

Lithosphere

Rigid outer layer consisting of crust and uppermost mantle; cools primarily by conduction.

Mantle Convection

Slow circulation of mantle material driven by heat from the core and radioactive decay; transports heat efficiently and drives plate tectonics.

Geotherm

The temperature profile of Earth’s interior as a function of depth.

Geodynamo

The process by which convection of liquid iron in the outer core generates Earth’s magnetic field.

Magnetic Dipole Field

Earth’s magnetic field behaves approximately like a tilted dipole, with magnetic poles offset ~10° from geographic poles.

Paleomagnetism

The study of fossil magnetism recorded in rocks (often magnetite), revealing past magnetic field orientations and reversals.

Magnetic Reversal

A switch in Earth’s magnetic polarity where magnetic north and south poles exchange positions; occurs on average every ~300,000 years.

Magnetite

An iron oxide mineral (Fe₃O₄) that records Earth’s magnetic field direction at the time of rock formation.

Magnetic Time Scale

A record of geomagnetic reversals extending back ~60 million years, used in plate tectonic reconstructions.

Aurora

Light displays near polar regions caused by interaction of charged solar wind particles with Earth’s magnetic field and atmosphere.

Seismic Tomography

A 3-D imaging technique using seismic wave travel times from multiple earthquakes to map temperature and compositional variations inside Earth.

Seismic Tomography – Hot vs Cold

Hot mantle regions slow seismic waves; cold subducted slabs increase wave velocity.

Farallon Plate

A subducted oceanic plate beneath North America identified in seismic tomography as a cold, high-velocity anomaly extending deep into the mantle.

Hot Spot Volcanism

Volcanism caused by mantle plumes rising from deep mantle; example

Core-Mantle Boundary (CMB)

Boundary between mantle and outer core marked by dramatic velocity changes and responsible for P-wave refraction and shadow zones.

Moment of Inertia Evidence

Earth’s rotational properties indicate most mass is concentrated near the center, supporting existence of dense iron core.

Why S-waves Cannot Travel Through Outer Core

Shear waves require rigidity; liquids cannot sustain shear stress, so S-wave velocity = 0 in outer core.

Why Inner Core is Solid

Despite high temperatures, extreme pressure forces iron atoms into a solid crystalline structure.

Mantle Plume

Upwelling column of hot mantle material that may originate near the core-mantle boundary and produce hotspot volcanism.

Building Blocks of Earth

Chondritic meteorites provide evidence for Earth’s original bulk composition, including iron-nickel core formation.

Pressure vs Temperature in Velocity Control

Pressure generally dominates velocity increases with depth because compression stiffens rocks; temperature effects are secondary in most mantle regions.

Core Solidification and Geodynamo

Gradual solidification of the inner core releases latent heat and light elements, driving outer core convection and sustaining the magnetic field.